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    SubjectRe: [patch/rft] jbd2: tag journal writes as metadata I/O
    Jan Kara <> writes:

    > Hi,
    >> In running iozone for writes to small files, we noticed a pretty big
    >> discrepency between the performance of the deadline and cfq I/O
    >> schedulers. Investigation showed that I/O was being issued from 2
    >> different contexts: the iozone process itself, and the jbd2/sdh-8 thread
    >> (as expected). Because of the way cfq performs slice idling, the delays
    >> introduced between the metadata and data I/Os were significant. For
    >> example, cfq would see about 7MB/s versus deadline's 35 for the same
    >> workload. I also tested fs_mark with writing and fsyncing 1000 64k
    >> files, and a similar 5x performance difference was observed. Eric
    >> Sandeen suggested that I flag the journal writes as metadata, and once I
    >> did that, the performance difference went away completely (cfq has
    >> special logic to prioritize metadata I/O).
    >> So, I'm submitting this patch for comments and testing. I have a
    >> similar patch for jbd that I will submit if folks agree that this is a
    >> good idea.
    > This looks like a good idea to me. I'd just be careful about data=journal
    > mode where even data is written via journal and thus you'd incorrectly
    > prioritize all the IO. I suppose that could have negative impact on performace
    > of other filesystems on the same disk. So for data=journal mode, I'd leave
    > write_op to be just WRITE / WRITE_SYNC_PLUG.

    Hi, Jan, thanks for the review! I'm trying to figure out the best way
    to relay the journal mode from ext3 or ext4 to jbd or jbd2. Would a new
    journal flag, set in journal_init_inode, be appropriate? This wouldn't
    cover the case of data journalling set per inode, though. It also puts
    some ext3-specific code into the purportedly fs-agnostic jbd code
    (specifically, testing the superblock for the data journal mount flag).
    Do you have any suggestions?


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